1. Field of the Invention
The present invention pertains to an apparatus and method for increasing the concentration of dissolved oxygen in a body of sewage, and more particularly it pertains to such means and methods which conserve oxygen from the supply from which the oxygen is drawn.
2. Description of the Prior Art
In the treatment of sewage by the conventional activated sludge process, the bacterial colony known as the bio-mass converts the carbon-hydrogen compounds within the sewage to additional bio-mass, carbon dioxide and water. Oxygen is an essential ingredient in this treatment process. During the course of the process enough oxygen must be present within the body of sewage to oxidize essentially all of the biodegradable matter contained therein.
Pressurized oxygenization is a concept for supplying oxygen to the bio-mass in the mixed liquor in an activated sludge sewage treatment system. Contacting (e.g., by bubbling methods) either air or oxygen with the mixed liquor at atmospheric pressure is a costly operation in the treatment of sewage, and the pressurized oxygenization concept offers the possibility of significant savings where oxygen is used. In patent application Ser. No. 822,594, filed Aug. 8, 1977, now U.S. Pat. No. 4,171,263, for "Presurized Oxygenation of Sewage," which is incorporated herein by reference, it is explained that the rate at which oxygen is dissolved in water at any given instant is a function of oxygen-water interfacial contact area and the difference between the concentration of oxygen in the water at saturation and the concentration therein at that instant. In the conventional treatment of sewage with air where oxygen in the air is to be dissolved in the sewage, the interfacial area is increased by agitation of the sewage. The difference at one atmosphere between the concentration of oxygen in the sewage at saturation and at a particular instant is generally 7 or 8 mg. of oxygen per liter of sewage. The agitation of the sewage must be sufficient to dissolve the required oxygen fast enough in the body of sewage to meet the demand of the bio-mass. When the difference between the concentration of oxygen at saturation and the concentration within the body of sewage at the instant is increased, such as by pressurizing the body of sewage, the agitation requirement is lessened, thereby reducing the energy required to oxygenate a given quantity of liquor within a given time.
Oxygen concentration at saturation, as discussed in the foregoing, is proportional to the partial pressure of the oxygen in the contacting gas. The oxygen partial pressure in air at atmospheric pressure is 0.21 atmospheres, and the concentration of oxygen in water at saturation is about 9 mg. per liter at 70.degree. F. When pure oxygen is used to contact a liquid, partial pressure being 1 atmosphere, the concentration at saturation is about 43 mg. per liter, and the rate at which the oxygen enters the liquid is about five times as great as when the partial pressure is 0.21 atmospheres, other factors being equal. Recognizing that the cost for agitating the body of sewage is reduced when using pure oxygen in contact therewith, it must also be recognized that the cost of producing oxygen from the air to obtain pure oxygen usually offsets the agitation cost reduction. The aforementioned patent application Ser. No. 822,594, now U.S. Pat. No. 4,171,263 for "Pressurized Oxygenation of Sewage", in FIG. 4 thereof, shows a sewage treatment system having an oxygen absorber and a pair of cylindrical lock-chambers. A system of check valves provides for alternate filling of the lock-chambers by sewage influent introduced through one end of each of the chambers. A flow separator movable along the length of each lock chamber is thereby forced to the opposite end of the chamber when sewage influent enters the chamber. When the flow separator is at the opposite end of a lock-chamber, a valve is controlled to communicate the oxygen absorber with the opposite end, and the lock-chamber is filled with oxygenated influent from the absorber thereby forcing the flow separator back to the one end of the lock-chamber and simultaneously expelling the sewage influent therein to the oxygen absorber. When the flow separator reaches the one end of the lock-chamber, the valve at the opposite end is then controlled to communicate the opposite end with the body of the sewage. The charge of oxygenated influent within the chamber is then expelled to the body of sewage when the lock-chamber is again filled from the one end with sewage influent. Great care is taken to avoid inducing turbulence in the oxygenated influent when the pressure thereon is released prior to expulsion to the body of sewage as well as during the subsequent transfer from the lock-chamber to the body of sewage. Alternate operation of the two lock-chambers provides a practically continuous flow of supersaturated oxygenated influent from the lock-chambers to the body of sewage. Large bubbles of oxygen which come out of solution from the oxygenated influent within the lock-chambers may be retained therein, providing a compressible fluid within the lock-chambers and therefore requiring additional energy to compress the fluid during the filling of the lock-chambers. Alternatively the large bubbles of oxygen coming out of the oxygenated solution at atmospheric pressure may be transferred with the oxygenated influent to the body of sewage, whereupon they rapidly rise to the surface and are lost before the oxygen has had sufficient contact time with the sewage.
U.S. Pat. No. 3,926,588 for a "Pressurized Transfer System" issued to Speece discloses a system for enhancing the absorption of gas by a fluent material while it is circulated alternately through two closed loops. Flow chambers are disposed in each of the closed loops, within which the fluent material is gasified during each half cycle. During the alternate halves of the cycles, the flow chambers are switched out of the closed loops so that the gasified fluent material therein may be displaced by flow thereto from the source of the fluent material. The flow from the source is under a lower pressure than the pressure which is imposed upon the fluent material during the half cycle when the fluent material is trapped in a closed loop.
The Speece apparatus provides unidirectional flow through each of the flow chambers thereby expelling any gas which comes out of solution each time a flow chamber is purged. Speece envisions such apparatus as useful for the purpose recited in the description of FIGS. 2A and 2B in column 4 of the specification. The Speece specification therein depicts a system wherein air as the gas may be dissolved into a quantity of liquid so that supersaturation and effervescence occurs when the liquid is depressurized and expelled from the flow chambers. The phenomenon is said to be useful in the separation of solids by flotation as well as for oxygenation purposes even though undissolved oxygenated gas is lost from the system.
U.S. Pat. No. 3,530,990, issued to Grimshaw, discloses a system for treating sewage which includes a tank within which are disposed one or more circulator units which receive a segregated quantity of influent directly from the tank. The segregated quantity is subjected to pressure while air is bubbled therethrough to thereby saturate the segregated quantity with air. The segregated quantity is then discharged into the body of sewage to accelerate promotion of bacteria and thereby expedite the treatment of the sewage. The Grimshaw disclosure explicitly envisions discharge of undissolved gas with the oxygenated quantities of influent in the body of sewage.
Several patents deal with dissolving molecular oxygen in a quantity of liquid apart from a main body of liquid and thereafter mixing the oxygenated quantity of liquid with the main body of liquid to enhance the dissolved oxygen content thereof. U.S. Pat. Nos. 3,617,537, issued to Vermette, and 3,799,511, issued to Svantesson, are representative. The Vermette disclosure cites a system for recirculating some sewage influent and dissolving molecular oxygen therein under pressure during recirculation. Turbulence is induced in the recirculating flow to assist in enhancing interfacial contact between the molecular oxygen and the recirculated influent. Oxygen injected into the continuous recirculation flow which is not dissolved therein will be discharged from the main body of sewage into which the oxygenated recirculated flow is injected.
The disclosure in the Svantesson patent discusses the dissolution of molecular oxygen in a liquid by mixing the oxygen with the liquid and subjecting the mixture for an adequate length of time to a pressure such that the oxygen dissolves in the liquid. The dissolved oxygen and liquid mixture is thereafter injected without any significant change in pressure into a body of liquid in which the dissolved oxygen content is to be elevated.
The Svantesson apparatus does not contemplate structure or method steps necessary to inject a supersaturated gasified liquid into a body of liquid with substantially no loss of gas from the solution.
Apparatus and method are desirable for use in oxygenation of bodies of sewage which conserve the molecular oxygen to be dissolved therein.